In seismic exploration, a large number of seismic sensor arrays are distributed along the earth's surface. Each sensor array may include from one to more than 30 individual sensors electrically connected together to form a single electrical element. Usually spaced at regular intervals, the arrays are disposed along a desired survey line at increasingly greater distances from a multichannel recording apparatus. The number of sensor arrays may range in number from a few tens to several hundred, depending upon the geological conditions. For many applications, fifty to one hundred such sensor arrays are typical. The distance or spread length between the first sensor array nearest the recording apparatus and the most remote array is frequently as much as two miles, although for certain types of exploration problems, the spread may be substantially shorter.
The sensor arrays are connected to the recording apparatus by means of a multiconductor cable or a telemetric link. Each array constitutes a single source of analog signals The respective arrays are connected to corresponding input channels of the multichannel recording apparatus.
Each input channel includes a preamplifier and a number of filter sections connected in cascade, the output of which is coupled to the corresponding input of a multiplexer switch. The multiplexer switch sequentially connects each channel to common signal processing electronics. The signal processing electronics samples the analog signals from the respective channels that are present on the multiplexer output bus, gain conditions the samples, converts the analog samples to binary numbers, and records the binary numbers on magnetic tape.
Each sequence of connecting each of the respective input channels to the common electronics is termed a scan cycle. The period of time required to complete one scan cycle is the sample interval. The reciprocal of the sample interval or the number of intervals per second is, or course, the sample rate. A sample interval typically ranges from 1 to 4 milliseconds (thousandths of a second). The corresponding sample rate is therefore 1000 to 250 samples per second.
In a seismic survey operation, a recording cycle consists of initiating a seismic impulse or acoustic wave at or near the earth's surface. The energy resulting from the seismic impulse travels downwardly into the earth and is reflected from various subsurface earth layers (strata). The reflected seismic energy returns to the earth's surface where it is detected by the seismic sensor arrays and converted to analog electrical signals. As outlined above, the detected analog signals are filtered, sampled, processed, and recorded. The entire recording cycle lasts from 6 to 10 seconds or more, depending upon the depth of the deepest stratum of interest (depth of penetration) and upon the type of seismic impulse generator used. Assuming 1-millisecond (ms) sample intervals, there would be six to 10 thousand scan cycles per recording cycle.
Many types of seismic impulse generators create energy having a broad frequency spectrum that extends from a few hertz (cycles per second) to several kilohertz (thousands of cycles per second). As is well known to geophysicists, seismic waves in the upper end of the seismic spectrum become attenuated (weakened) as the weaves travel deeper into the earth. The energy loss is due to solid friction in the earth and to scattering by inhomogeneities such as boulders and and fractures in the subsurface strata. Accordingly, seismic energy from relatively shallow strata, 500 to 1000 feet deep, may be rich in high frequencies up to 1 or 2 kilohertz. On the other hand, seismic energy reflected from deep-seated strata is characterized by much lower frequencies in the 10-30 hertz range.
In seismic exploration, the stratigraphic resolution, (the capability of distinguishing two closely spaced strata or earth layers) depends, among other things, upon the frequency of the reflected seismic signals (reflections). Use of relatively high-frequency reflections in the range of 100 to 1000 Hz (hertz) is needed to distinguish between layers that are separated by only a few feet. Stratigraphic resolution of the above-defined degree is required for study of shallow strata in connection with engineering foundation problems, coal exploration, etc. In petroleum exploration, higher resolution is sought at all levels but is more critical at the shallower levels because smaller deposits may be of interest in view of the lower cost of drilling shallower oil wells. Grosser resolution of deep-seated strata may be found to be acceptable. Hence, lower-frequency reflections can be useful for deeper exploration.
As outlined above, the reflected seismic signals are converted to an oscillatory analog wave train by the seismic sensor arrays. In the seismic signal processing and recording apparatus, the analog wave trains are sampled at selected time intervals, converted to binary numbers, and recorded on magnetic tape. At some later time, in a data processing center, the original analog wave trains are reconstructed from the tape-recorded, binary, data samples. In accordance with the well known "Nyquist Theorem," if it is desired to transmit a signal of a predetermined frequency or containing a predetermined band of frequencies, the sampling rate must be at least twice the highest frequency which is to be transmitted. In the course of the sampling process, input frequencies greater than the Nyquist frequency may produce spurious or so-called "alias" low-frequency signals which would be indistinguishable from the desired data signal information. Put another way, the Nyquist Theorem indicates that an oscillatory analog signal must be sampled at least twice during each complete wavelet cycle. If the signal is sampled less than twice each cycle, the reconstructed wavelet may exhibit a frequency that is different from the true frequency and may be otherwise changed in form. That is, the reconstructed frequency assumes a disguise or "alias". For example, if a 500-Hz signal were sampled exactly 500 times per second, such as at each positive peak, the signal samples would all have the same value. The reconstructed signal would have "zero" frequency. Accordingly, the original 500-Hz signal must be sampled at least 1000 times per second (1-ms sample intervals) in order preserve the frequency of the original 500-Hz signal. More than two samples per cycle are required to preserve the amplitude of the signal.
The filter cascades discussed above in connection with the preamplifiers include one or more "anti-aliasing" filters or, for brevity, simply "alias filters". The alias filter is designed to sharply attenuate (weaken) all frequencies greater than the alias frequency (1/2 the sampling frequency) corresponding to the selected sample rate. Characteristically, the alias filter attenuates frequencies equal to one-fourth the sampling frequency by 3 dB (1.4:1) at the "cutoff frequency", although some manufacturers specify 6 dB (2:1). Frequencies greater than one-half the sampling frequency are attenuated by about 80 dB (10,000:1). In the above example, the cutoff frequency is 250 Hz; at 500 Hz, the signal level is attenuated by nearly 80 dB.
In practical operations, the sample rate is adjusted in accordance with the highest frequency of the seismic signals of interest subject to the capacity of the system. Low-frequency reflections of 20 Hz for example, having a period (time to complete one full cycle) of 50 ms, can be sampled rather coarsely, such as at 4-ms sample intervals. High-frequency reflections of, for instance, 1000 Hz having a period of 1 ms, must be sampled at intervals substantially less than 0.5 ms and preferably less than 0.25 ms.
Any given seismic recording systems will be characterized by a maximum overall sampling rate. That maximum or fastest sampling rate is the maximum rate of the slowest of the various individual subsystems which make up the entire recording system. In most present-day systems it is the tape-recording unit which sets the overall limit, although the analog-to-digital converter is usually selected to have a capacity commensurate with that of the tape recorder. Typical maximum rates in the contemporary art are of the order of 25,000 or 50,000 samples per second.
There is some latitude in selecting the number of channels and the number of samples per second for each channel provided that the product of the two numbers does not exceed the maximum sampling rate of the system. A 100-channel system for example, will have a typical lower limit to the basic multiplexer scan-cycle time of 2 ms, corresponding to a maximum sample rate of 50,000 samples per second.
The typical methods of seismic reflection surveying involve linear arrays of detectors with the seismic source off one end of the line. Accordingly, a substantial time elapses after the shot before useful reflection signals reach the detectors that are most distant from the location of the shot. Thus, only a fraction of the detectors nearer the shot point actually receive useful signals for a certain initial period of time. The number of detectors receiving useful signals increases with increasing elapsed time after the shot, until finally all detectors are usefully active.
Present-day seismic recording equipment used for petroleum exploration provides for two or three, switch-selectable sample intervals, such as 1, 2, 4 ms or 2, 4, 8 ms. An alias filter, appropriate to the sample interval selected, is switched into the input circuit when the sample interval is changed. Once a sample interval has been set, it cannot be changed during a recording cycle. To do so would introduce severe switching transients by injecting a step function into the filters at the time of switch-over which, of course, is intolerable. The step function is due to the high probability that switching will occur at a time that the incoming data signal has a value other than zero.
Conventional petroleum-exploration type recording apparatus can be modified for high-frequency recording by using fewer channels, but only after making substantial, semi-permanent wiring modifications to the apparatus. Once modified, rewiring is required to revert to a coarser sampling interval. Accordingly it is impractical to make such a change between recording cycles, much less within a recording cycle.
Use of a time-varying sampling rate was contemplated and discussed in co-pending applications Ser. Nos. 665,150 and 665,151, assigned to the same assignee. In these applications, use of different-length multiplexer scan-cycle times was suggested. In the above-cited applications, the alias filters were not changed when the sample rate was changed.
As will be discussed in detail later, a spread length that may be suitable for exploration of deep-lying strata is usually too long to properly record reflections from shallow strata. Using existing equipment, there is no way to alter the spread length during a single recording cycle.